Solar wind hammers the ozone layer

A stream of particles from the Sun, in combination with extreme weather conditions, caused an unprecedented thinning last year of the upper Arctic ozone layer.

Scientists have been puzzled by the chemical processes that destroyed up to 60% of ozone molecules in the lower mesosphere and upper stratosphere (the atmospheric layers that lie 30 to 40 kilometres above ground) in the first months of 2004. Reactions with chlorofluorocarbons (CFC), the compounds responsible for ozone depletion in the lower stratosphere, could not explain the decline in higher layers.

Now an international team of atmosphere researchers, led by Cora Randall of the University of Colorado at Boulder, has suggested a natural cause for this ozone loss at high altitudes.

Strong solar storms in October 2003 carried energetic electrons and protons into the Earth's upper atmosphere, where they boosted production of nitrogen oxides by a factor of four. Such oxides are a known group of ozone killers. Very strong winds inside the polar stratospheric vortex, which was exceptionally powerful last winter, then transported the excess nitrogen gases further into the atmosphere. At around 40 kilometres' height, they mixed with, and attacked, the ozone layer.

Shaky shield

“That we can still be surprised illustrates the difficulties in separating atmospheric effects due to natural and human-induced causes”

Cora RandallUniversity of Colorado, Boulder

Ozone is a form of oxygen that shields the Earth from dangerous ultraviolet radiation from space. Ozone holes were first detected in the 1980s above the South Pole. Soon afterwards, CFCs were phased out under the 1987 Montreal Protocol. Ozone holes do still occur regularly in the Antarctic, but at high northern latitudes they are observed only in particularly cold winters1.

"No one predicted the dramatic loss of ozone in the upper stratosphere of the northern hemisphere in the spring of 2004," says Randall. "That we can still be surprised illustrates the difficulties in separating atmospheric effects due to natural and human-induced causes."

The team analysed data from satellite instruments that measure nitrogen oxides and ozone in the atmosphere. Nitrogen-oxide-rich air first began to descend in January, and led to strong ozone depletion between March and May. The effect was still present in July, but declined as the stratospheric vortex broke up later in the year.

Whether the full magnitude of the anomaly can be explained by the extra injection of solar particles must still be investigated, says Randall. The research is published online in Geophysical Research Letters2.

Cold comfort

Determining the influence of solar particles on the composition of the atmosphere is crucial for understanding the links between solar activity, ozone depletion and climate, says Peter von der Gathen, an atmospheric physicist at the Alfred Wegener Institute in Potsdam, Germany.

This research shows, for the first time, a long-term atmospheric influence of solar activity in the Arctic, says von der Gathen. The next step, he adds, is to determine the influence of solar-particle fluxes on the chemistry of the atmosphere over a longer period of time. Such a study, based on a 14-year data series, is currently under way in Germany.

Such work might help explain why existing atmosphere models have not matched observed ozone losses during several cold Arctic winters over the past decade.

Despite strong depletion in the upper ozone layer, a full-fledged ozone hole did not occur in the Arctic last year, thanks to relatively warm temperatures. But although solar activity has now calmed down, scientists expect significantly larger ozone losses this year, which has so far been exceptionally cold.